The fetal and maternal concentration of various plasma proteins alters during pregnancy. Cells in the livers of fetal hamsters accumulate serum amyloid A (SAA) and C-reactive protein (CRP) mRNA, major acute phase reactants, when lipopolysaccharide is administered to the fetal circulation. No fetal SAA or CRP mRNA response is seen when the mother is stimulated at a remote site by endotoxin or a nonspecific inflammatory agent. In addition, cells of the fetal hamster liver do not respond by accumulating SAA mRNA when exposed to the specific cytokines, tumor necrosis factor, IL-1, and IL-6. CRP mRNA levels increased in fetal livers after administration of tumor necrosis factor and IL-1. These data suggest that cells contained in the fetal liver can respond during an acute phase reaction but that the capacity of some acute phase reactant genes to respond to cytokines may be developmentally regulated. Studies of immature hamsters after birth show that the responses of CRP and SAA genes to lipopolysaccharide, tumor necrosis factor, IL-1, and IL-6 are reduced when compared with induction of mRNA accumulation for these acute phase reactants in adult animals.